The present invention relates to a plasma generating and processing method and to an apparatus for use in the method.
As an arrangement for practicing a plasma generating and processing method, there has been known one in which a sample stage serving as a cathode electrode is disposed in a chamber of a plasma generating apparatus. With the arrangement, high-frequency power is applied to the sample stage to form a dc self-bias, thereby accelerating ions toward the sample stage. The accelerated ions are used either primarily or subordinately to process a sample on the sample stage.
The plasma generating and processing method using high-frequency discharge has various applications in such fields as plasma CVD, sputtering, and ion implantation, which are used in dry etching techniques and thin-film forming techniques that necessitate micro-processing.
Recent advances in high-density semiconductor integrated circuit devices have been bringing about changes comparable to those caused by the Industrial Revolution. Higher densities have been achieved in semiconductor integrated circuits by device miniaturization, device improvements, and an increasing area occupied by a chip. Among these, device miniaturization has reached a point on the order of the wavelengths of light, so that the use of excimer laser and soft X-rays has been considered. In forming micro-patterns, dry etching and thin-film formation as well as lithography have been playing an important role.
Below, an examination will be made of dry etching process applied to micro-processing techniques for semiconductor substrates.
The dry etching process is a technique for removing unwanted portions of a thin film or a semiconductor substrate, which serves as a material to be etched, by utilizing chemical or physical reactions between radicals and ions generated by a plasma and the surface of a solid phase of the material to be etched. Of all the dry etching techniques, reactive ion etching (RIE) is most widely used. In RIE, a sample is exposed to a plasma obtained by subjecting an appropriate reactive gas to high-frequency discharge, so that unwanted portions at the surface of the sample are removed by etching reactions. The wanted portions at the surface of the sample are normally protected by a photoresist pattern used as a mask.
In main etching, to realize substantially vertical etching profiles with complete fidelity to a minuscule mask pattern at a high etching rate, it is required that large ion fluxes should be incident at high energy upon a sample in directions substantially vertical to a surface thereof. To meet the requirement, it is essential to minimize the number of times that ions in the plasma are scattered in collision with neutral particles, while they are accelerated toward the sample stage in a sheath region formed in the vicinity thereof.
In overetching, on the other hand, it is required that the ion fluxes should not be incident at high energy upon the sample in directions substantially vertical to the surface thereof in order to enhance the etching selectivity to an underlying material.
To fulfill the above requirements, there has conventionally been adopted an arrangement in which the power of a high-frequency power source is increased in main etching, while it is reduced in overetching or another arrangement in which a gas for forming a protective thin film on the surface of the underlying material is additionally introduced at the time when the underlying material is exposed, thereby performing two-level etching.
Next, an examination will be made of thin-film forming process using plasma CVD, which is applied to micro-processing techniques for semiconductor substrates.
In forming a thin film by directing reactive products generated in the plasma, such as neutral radicals, to the sample on the sample stage, it is generally required to form a uniform thin film over the surface of the sample with concave and convex portions that are either rectangular or circular in cross section.
However, a thin film deposited on the surface of the sample with concave and convex portions becomes thickest at the upper edge corners of a concave portion, since the prospective landing solid angle at which the reactive products, such as the neutral radicals, land on the surface on the sample in substantially conformal directions becomes largest there. Conversely, the deposited film on the surface of the sample becomes thinnest at the lower edge corners of a convex portion, since the prospective landing solid angle becomes smallest there. Moreover, the deposited film becomes comparatively thick on the surface of the sample, since the prospective landing solid angle is comparatively large there. The prospective landing solid angle becomes particularly small at the bottom of the convex portion at the surface of the sample, since the deposited film formed on the periphery of the convex portion is jutting out over the upper edges thereof, so that the deposited film becomes thin at the bottom. Accordingly, the deposited film is filled in the convex portion only unsatisfactorily, which often results in voids.
To prevent the above phenomenon and form a uniform thin film on the surface of the sample with rectangular and circular concave and convex portions, there has conventionally been adopted a method in which, after the thin-film forming process was performed for a specified period of time, a sample is taken out of a thin-film forming apparatus and temporarily placed in a sputtering apparatus, so that those portions of the deposited film which are jutting out, in the course of the thin-film forming process, from the upper edge corners of a concave portion at the surface of the sample are removed by ion sputtering, thereby increasing the prospective landing solid angle at the bottom of the convex portion of the sample. Thereafter, the semiconductor substrate is placed in the plasma CVD apparatus again. Thus, by placing the sample alternately in the thin-film forming apparatus and the sputtering apparatus and alternately performing the thin-film forming process and the ion sputtering process, a thin film uniform in thickness is formed over the surface of the sample with concave and convex portions with excellent step coverage.
However, although the above etching method in which the power of the high-frequency power source is switched between two levels can control the density of ion fluxes, it cannot control the energy distribution of ions and the incidence angle distribution thereof satisfactorily, so that it is difficult to obtain a sufficiently high etching rate, substantially vertical etch profiles, and sufficiently high selectivity. A combination of the method in which the power of the high-frequency power source is switched between two levels and the method in which a gas is additionally introduced in overetching also has similar disadvantages.
Furthermore, since the process of forming a thin film by plasma CVD and the process of removing the thick portions of the film by sputtering are performed individually in the different apparatus, throughput in processing is significantly reduced.